The inexorable progress to the ultimate miniaturization of electronic devices provides the impetus for development of molecular electronics. The ultimate goal of research in molecular electronics is the utilization of single molecules as functional electronic elements and their integration into more complex devices. In comparison with the conventional semiconductor devices, molecular electronic devices can be smaller, faster, and dissipate less energy during operation. Moreover, the diversity of molecular materials presents an abundance of functional elements and concepts for construction of single molecule devices based on theoretical modeling and atomic scale architectonics employing chemical synthesis, molecular self-organization, and nanofabrication. Following the first proposal of a unimolecular rectifier with the D-G-A structure, a variety of the elementary electronic devices based on single molecules, including switches, diodes and transistors, have been conceived and demonstrated.
As the elementary units of logic gates and electronic memories, molecular switching devices are arguably the simplest and most fundamentally important molecular electronic components, which have been investigated with vigor since their first proposal. A typical molecular switching device consists of a junction formed by two electrodes forming an electrical contact to a single molecule active element. The current flowing through the junction is controlled by the molecular configuration, and can be switched reversibly among different states under various stimuli, such as electron charge, electric field, light, short-range chemical force, heat, and magnetic field. Common embodiments of single molecule switches are a controllable break-junction or a tunneling junction formed by metal substrate and tip within a scanning tunneling microscope (STM). In the former case, functionalized molecules chemically attach to each electrode, whereas in the latter case, although molecules can also be attached to both electrodes, usually the STM tip forms a tunneling barrier, and even the substrate can consist of a thin insulating layer to form double barrier junction.
The first demonstration of a molecular level switch was reported for an STM junction consisting of a Xe atom between a nickel substrate and a tungsten tip. By applying voltage pulses with opposite polarity, single Xe atoms could be reversibly transferred between the tip and the substrate, with the concomitant change in conductivity. Because the conductivity is modulated by changes in the electrode through interaction with the atomic/molecular element rather than within the molecule itself, such a switch is considered to be extrinsic. Other extrinsic switches have incorporated oligophenylene-ethynylene derived molecules, bipyridine, C60 and H2. By contrast, intrinsic single molecule switches utilize the specific switching behavior of the molecule itself.
The variety of molecular materials offers much larger range of intrinsic switching behavior employing multiple conformational, charging and magnetic mechanisms. Because the switching mechanism can be designed by chemical means, the conformational switching among isomers has been widely studied. Inspired by the primary switching events in vision and bacterial photosynthesis, which rely on the conformational change through cis-trans isomerization, single-molecule switches using azobenzene and its derivatives and analogues have been obvious targets of research. Whereas tunneling electron induced switching is found in various azobenzene derivatives, reports of related photoswitching are rare, the reason being that the switching quantum yields are extremely small, and electrons can be delivered by STM specifically to a single molecule within a junction, whereas photons irradiate much wider area. The quantum yields are small because the strong chemical and electronic interactions of molecules with the substrate can sterically hinder the isomerization and quench the electronically excited states of the molecule. To ameliorate these difficulties, prior work modified the pristine azobenzene by functionalizing the benzene ring with tert-butyl side groups. The four added ‘legs’ lift the active elements of azobenzene molecules from the surface to enable the photoswitching. In another approach, the photoswitching by incorporating azobenzene derived molecules into self-assembled monolayer of dodecanethiols was demonstrated. The derivatized molecules adopt a vertical-standing structure with a short alkane spacer separating the azobenzene and the substrate. The upright molecules therefore have more degrees-of-freedom for conformational change and are also more weakly coupled to the substrate than recumbent ones.
A disadvantage of switches that are based on a large structural change of single molecules is that it is difficult to track the conformational changes with an STM tip in order to actuate and record the switching behavior over multiple switching cycles, as might be essential for a practical operation of a single molecules switch. This problem is compounded when combining individual molecules to create devices and circuits with more complex functionality, where molecular conformational changes may not be conducive to stable device performance. Unfortunately, most of the single molecule switches that have been realized so far in this category are based on molecules that undergo considerable structural changes, such as the catenane, rotaxane, diarylethene and porphyrin derivatives. Other switches, for example, based on the hydrogen atom tautomerziation in naphthalocyanine, or the mechanical oscillation of the N—H bond in an engineered melamine, minimize the overall structural change, but their functional N—H groups are vulnerable to external chemical perturbations that can deactivate the function, such as hydrogen bond formation with impurity molecules like H2O. The discovery of molecular switches with minimal structural change, strong chemical stability with respect to switching, and adamant to environmental perturbations, therefore, remains an important step towards achieving practical single molecule switches.